1. Field of the Invention
This invention relates to semiconductor device manufacturing, and more particularly, to an improved method and system for removing matter adhered to a polishing pad.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Fabrication of an integrated circuit involves numerous processing steps. For example, after implant regions (e.g., source/drain regions) have been placed within a semiconductor substrate and gate areas defined upon the substrate, alternating levels of interlevel dielectric and interconnect may be placed across the semiconductor topography to form a multi-level integrated circuit. Such a multi-level integrated circuit may include a plurality of layers and structures. Forming substantially planar upper surfaces of the semiconductor topography during intermediate process steps of the process may facilitate fabrication of layers and structures that meet design specifications. More specifically, forming a substantially planar surface may aid in forming layers and structures that meet the elevational and lateral design specifications of subsequently formed semiconductor devices.
Forming substantially planar upper surfaces during intermediate steps of a fabrication process may play an important role in the functionality of a semiconductor device. For example, problems with step coverage may arise when a dielectric, conductive, or semiconductive material is deposited over a topological surface having elevationally raised and recessed regions. Step coverage is defined as a measure of how well a film conforms over an underlying step and is expressed by the ratio of the minimum thickness of a film as it crosses a step to the nominal thickness of the film over horizontal regions. Furthermore, substantially planar surfaces may become increasingly important as the feature sizes of semiconductor devices are reduced, since the depth of focus required to pattern an upper surface of a semiconductor topography may increase with reductions in feature size. If a topography is non-planar, the patterned image may be distorted and the intended structure may not be formed to the specifications of the device. Furthermore, correctly patterning layers upon a topological surface containing fluctuations in elevation may be difficult using optical lithography. The depth-of-focus of the lithography alignment system may vary depending upon whether the resist resides in an elevational “hill” or “valley” area. The presence of such elevational disparities therefore makes it difficult to print high resolution features.
One manner in which to reduce elevational disparities of layers and structures formed during intermediate steps of a fabrication process is by polishing the layers and structures. Such a process may be performed by a fixed abrasive polishing process or a process referred to as chemical-mechanical polishing (“CMP”). A conventional polishing process may involve placing a semiconductor wafer face-down on a polishing pad which lies on or is attached to a backing structure. During the polishing process, the polishing pad and/or semiconductor wafer may be set in motion as the wafer is forced against the pad. For example, the polishing pad and the wafer may be placed on a rotatable table such that the wafer and the polishing pad may be rotated relative to each other. Alternatively, the wafer may be rotated relative to a fixed pad or vice versa. In another embodiment, the polishing pad may be a belt, which traverses against a fixed or rotating wafer. In either embodiment, the rotatable table, fixed pad, or belt may serve as the backing structure to which the polishing pad lies upon or is attached.
A fluid-based chemical suspension may be deposited onto the surface of the polishing pad as the pad and/or wafer is set in motion. The movement of the pad and/or wafer may distribute the fluid within the space between the polishing pad and the wafer surface such that debris polished from the surface of the wafer surface may be washed away. In a CMP process, the fluid is often referred to as a “slurry” and typically contains abrasive particles with which to physically strip the reacted surface material of the wafer. In this manner, the CMP process may employ a combination of chemical stripping and mechanical polishing to form a relatively level surface. Alternatively, the polishing fluid may be substantially absent of such abrasive particles, such as with fixed abrasive systems. In addition or alternatively, the pad itself may physically remove some material from the surface of the wafer. The polishing pad may include a textured upper surface with which to polish the topography. In addition, the polishing pad may include a plurality of pores dispersed across the entirety of the pad. The slurry applied to the polishing pad during the polishing process may fill the pores such that the majority of the fluid may be kept within the vicinity of the pad.
When used to planarize a semiconductor wafer surface, a polishing system has at least two important performance factors: (i) polishing removal rate, and (ii) resultant semiconductor wafer surface planarity or “uniformity”. A high polishing rate is desirable in order to maximize the number of wafers which may be planarized in a given amount of time. A high measure of resultant semiconductor wafer surface planarity or “uniformity” is desirable to reduce the step coverage and depth of focus problems described above. Such a measure of uniformity may be measured across a single wafer or between multiple wafers.
Unfortunately, the polishing rate performance of polishing systems and the resultant uniformity of wafers polished by such systems degrades as matter builds up in the pores and on the upper surface of the polishing pad during the polishing process. The matter may include particles from the polishing fluid or from the polished wafer. As the polishing chemistry is exposed to air during the polishing process, the liquid portion of the fluid evaporates leaving polishing solution particles and wafer particles to clog the pores of the polishing pad. The slurry particles, in particular, tend to agglomerate forming large masses adhered to the polishing pad. Such clogging restricts the amount of slurry that is able to fill the pores and consequently limits the amount of slurry that may be contained within the vicinity of the polishing pad. In addition or alternatively, the matter may accumulate or agglomerate upon the upper surface of the pad. Such an accumulation may be referred to as “glazing” and essentially smoothes out the textured surface of the pad, thereby reducing the effectiveness of the polishing pad. Consequently, the efficiency and performance of a polishing system may be adversely affected by matter adhered to the polishing pad of the system.
In order to increase the effectiveness of a polishing pad in a polishing system, the polishing pad may be cleaned periodically. Such a process is typically a sporadic manual process which involves shutting down the polishing system and depositing water upon the pad in an effort to suspend the particles in solution and subsequently wash them away. Unfortunately, such a process typically does not remove all matter from the pad. More specifically, the conventional cleaning process may only be able to suspend matter loosely adhered to the polishing pad. As such, the current cleaning process may not be able to dislodge all matter adhered to the polishing pad. Consequently, the polishing performance and efficiency of the system may degrade more quickly since additional matter may build upon the remaining matter. In addition, such a cleaning process is typically performed when the polishing system is not in use. Typically, in order to reduce downtime of the polishing system, the cleaning process is performed after a specific number (e.g., 25) of wafers has been processed. In this manner, as the polishing process continues, matter continues to accumulate upon the polishing pad and uniformity from wafer to wafer decreases. Furthermore, since the process is manual, the length and the coverage of the cleaning process may vary. As such, the performance and efficiency of the polishing system may vary, thereby reducing the process capability of the system.
Accordingly, it would be advantageous to develop a method and a system for removing matter adhered to a polishing pad during the use of a polishing system.